At least 500 people have marched silently through a suburb of Paris in memory of two teenage boys whose deaths a year ago sparked riots across France.

The crowd in Clichy-sous-Bois held a banner saying the two youths from immigrant families "died for nothing".

Interior Minister Nicolas Sarkozy ordered police to step up security on buses, after several were hijacked and set ablaze ahead of the anniversary.

Violence engulfed Clichy-sous-Bois and several other suburbs in late 2005.

The families of the two dead youths laid wreaths at the electricity sub-station where they were electrocuted. There has been dispute over whether they were running from police at the time.

A monument to the boys, Bouna Traore and Zyed Benna, was unveiled in Clichy-sous-Bois and the local mayor appealed for calm.

"Last year we crossed Clichy by weaving between the burnt-out wrecks of cars, creating an image of our city that we didn't want to see," said mayor Claude Dillain, quoted by the Associated Press.

"Once again France, and even the world, is watching us and waiting to see what we do. So I appeal solemnly for calm and dignity to prevail here."

Youths in Clichy-sous-Bois and other bleak suburbs on the edge of Paris told the BBC News website that the authorities should do more to tackle racial discrimination and high unemployment.

This time last year clashes erupted between youths of mainly North African origin and police in suburbs throughout France. In three weeks of violence more than 10,000 cars were set ablaze and 300 buildings firebombed.

'No publicity'

Mr Sarkozy, a conservative tipped to run for the presidency next year, said the authorities would "do everything possible to ensure that public services are not disrupted anywhere".

He said the media should act responsibly in reporting on the anniversary of last year's violence, to avert "copycat" crimes.

"We should not give any publicity to people who want nothing else," he said.

A security services report leaked to a French newspaper this week said that the conditions that led to last year's riots were still in place.

Law and order has become a major issue, with presidential elections due next April. Candidates from the two biggest parties have promised a tough approach to crime.